Your loan is assigned to a loan servicer by the U.S. Department of Education after the loan is first disbursed.
The only time in which you may request a new federal loan servicer is during a loan consolidation. During the application process you are able to select which servicer you would like for your new consolidation loan.
The U.S. Department of Education also has the ability to switch your servicer at any point if need be (i.e. when a specific servicer contract ends or when a borrower submits their first Employment Certification Form for PSLF). If you feel that your current loan servicer has mistreated you, you may submit a complaint to the Federal Student Aid Feedback Center with proof of your claim.